Optical output devices are widely used in navigation devices. Such output devices may be used for outputting electronic maps. Since electronic maps may be displayed on screens of small and/or portable devices, they have the advantage of being versatile and compact.
Three-dimensional (3D) maps, i.e. perspective representations, may be of particular value to the user owing to their high recognition quality. The recognition of an environmental region, such as a street intersection, may be facilitated when a three-dimensional map is output, as compared to a conventional two-dimensional representation.
Various approaches for configuring a navigation device such that 3D maps can be displayed are conceivable. Graphical images representing a perspective view of road intersection areas may be pre-computed and stored in the navigation device. Such an approach would be efficient in terms of computation time when the 3D map is to be displayed, but suffers from reduced flexibility. If a limited set of graphical images representing a finite set of intersection types is stored, integration of such graphical images with a terrain model may be challenging.
In another approach, digital terrain model (DTM) data may be stored in the navigation device. The navigation device may compute a projection of the two-dimensional road vectors of a conventional digital map onto the surface defined by the DTM data at run time. Such an approach provides higher flexibility, but may be costly in terms of computation time and/or computation resources. EP 1 107 189 A2 describes a map displaying system and method in which a navigation system stores a 2D map and landform data, which need to processed by the navigation system to prepare 3D map data.